Gains in soil C storage under anthropogenic N deposition are rapidly lost following its cessation

ABSTRACT Anthropogenic nitrogen (N) deposition enhanced the global terrestrial carbon (C) sink and partially offset anthropogenic CO 2 emissions. This was driven by the suppression of microbial activity associated with the breakdown of soil organic matter. However, since the implementation of emission abatement policies in the 1970’s, atmospheric N deposition has declined globally, and the consequences of this decline are unknown. Here, we assessed the response of soil C storage, and associated microbial activities, in a long-term field study that experimentally increased N deposition for 24-years. We measured soil C and N, parameters of microbial activity, and compared effect sizes of soil C in response to, and in recovery from, the N deposition treatment across the history of our experiment (1994-2022). Our results demonstrate that the accumulated C in the organic horizon has been lost and exhibit additional deficits 5-years post-termination of the N deposition treatment. These findings, in part, arise from mechanistic changes in microbial activity. Soil C in the mineral soil was less responsive thus far in recovery. If these organic horizon C dynamics are similar in other temperate forests, the Northern Hemisphere C sink will be reduced and climate warming will be enhanced.